DESCRIPTION OF THE PRIOR ART
Thermosetting resins in the form of condensates of a phenol or urea with formaldehyde have heretofore enjoyed widespread utility as adhesives in wood bonding applications for the construction industry. These adhesives in general exhibit excellent strength properties as well as good durability and water resistance. The disadvantages attendant to their use are essentially environmentally concerned. The indicated adhesive resins are utilized as an organic solvent solution in a manner whereby it is not feasible to recover the hydrocarbon solvent in the eventual curing or consolidation process. While such adhesives are generally cured at a relatively moderate elevated temperature, free formaldehyde is nonetheless generated in the curing process thus posing potential health problems for workers. These resins require costly and inconvenient heat curing. When cured they may not be sufficiently durable for exterior applications. This deficiency is in part due to their susceptibility to ultraviolet degradation.
Most recently a new area of adhesive technology based on urethane chemistry has been developed which obviates the indicated disadvantage associated with the aforementioned prior art compositions. The new adhesives are water-borne compositions in which the curable resinous matrix is essentially composed of a polyisocyanate and polyvinyl alcohol. Optionally, colloidally dispersed SBR and vinyl acetate-ethylene copolymers can be included in the compositions for enhancing adhesive properties. Beyond exhibiting adhesive strength properties and water resistance comparable to the phenolic adhesives, reduced brittleness of the resultant cured adhesive and better resistance to discoloration represent improved properties to be gained in their use. Moreover, they can be conveniently cured at ambient temperature as low as 10.degree. C. U.S. Pat. No. 3,931,088 is directed to adhesive compositions of this type.
As mentioned, the indicated prior art adhesives provide excellent bonding in wood-to-wood applications. However, in specialty applications wherein a flexible substrate, e.g., plastic sheeting or film is bonded to wood, the degree of adhesiveness at the plastic interface exhibited by such compositions is less than desired. It is, accordingly, the object of this invention to ameliorate this deficiency of the relevant prior art adhesives.